Assign VPN network to specific app

I would like to use my email client, Thunderbird, purely with my chosen VPN and have all rest of the system traffic route through my ISP as it normally would.

Can this be done with Comodo’s firewall?

I have looked at several methods to achieve this but am not sure what way to go about it.

Some people say to make a VM, connect that to the VPN and use the VM purely for emailing via the VPN. This however I think will be too much load for the system just for emails. Also I am not sure what VM to use and if I can go back and forth to the normal system without having to reboot all the time.

I have come up with something called split tunnelling. Perhaps that could work out? Use the VPN connection for the email traffic only and the rest for the normal system traffic. I have a network and a wireless card, so two cards, so perhaps that can also help doing split tunnelling. Would split tunnelling actually enable me to use Thunderbird on the VPN exclusively?

Another thing would be to use Proxifier. However that is not meant for VPNs but for proxies. So I guess that would fail as well, even though Proxifier can assign processes/apps to use certain proxies as opposed to the normal system traffic.

Last not least, would it be possible to set up network zones in Comodo firewall and assign those network zones to Thunderbird? How could I do this?

Many thanks for any input or any other way you could think of to get this done.